The concept of the magnetic heat pump is based on the principle of magnetocaloric effect of magnetic materials, in which entropy, and therefore temperature, changes when a material is magnetized or demagnetized. When a magnetic material is in its natural (i.e., zero magnetic field) state, the magnetic dipoles in the material are in a relatively disordered state.
If a magnetic field is imposed upon the material, the dipoles align with the field and are transformed into an ordered state, and a decrease in entropy (corresponding to an increase in temperature) occurs. Conversely, if a magnetic material is suddenly demagnetized by being removed from a magnetic field, an increase in entropy and a corresponding decrease in temperature will occur.
The concept of magnetic cooling can be traced back to the 1920's, when Giauque and Debye independently proposed using the magnetocaloric effect of magnetic materials for refrigeration to produce ultra-low temperatures. Giauque used a method in which a paramagnetic salt was cooled to 3.5 K in a magnetic field and then demagnetized adiabatically to achieve 0.5 K. This adiabatic demagnetization method is a one-shot or single-step refrigeration process that does not provide continuous cooling. While failing to achieve wide-spread commercial application, the Giauque method is still being used in low-temperature physics experiments to create temperatures extremely close to absolute zero.
The possibility of building a heat pump using the magnetocaloric effect was believed to have been first suggested by J. G. Daunt and C. V. Heer in Physics Review, 76, 854 (1949), who combined two isothermal and two adiabatic magnetization and demagnetization processes to form a magnetic Carnot heat pump cycle that was capable of providing the sustained cooling. However, the laboratory experimentation was not performed until 1975, when Brown built and tested a reciprocating magnetic heat pump assembly using gadolinium as the working medium. Brown'w work was published in the Journal of Applied Physics, 47(8), 3674 (1976).
Like the magnetic properties of materials, the temperature change caused by the magnetocaloric effect is highly dependent upon a strong magnetic field. Strong fields created by superconducting magnets are often preferred and are probably necessary for many practical applications. The complexity and relatively high cost of the traditional superconducting magnets (which must be cooled by liquid helium) are among the factors that have affected interest in magnetic heat pump development. The discovery of high-temperature superconductivity shows promise for achieving not only higher magnetic fields than before but also for being a simpler and less costly option (which may be cooled by liquid nitrogen or gaseous helium, for example). The continued advancements made in developing new superconducting materials is expected to enhance the viability of magnetic heat pump technology as well.
FIG. 1 schematically represents one type of magnetic refrigeration apparatus 10 in which a magnetocaloric material 12 is disposed within a pulsed NbTi superconducting solenoidal magnet 14. Power source 16 creates a cycling magnetic field between 0 and 7 tesla. The material 12 is made of Gadolinium alloys and requires a pulse magnet with a rapid duty cycle (such as 10 cycles per minute) to as high a field as possible so that the Gadolinium material can be constantly magnetized and demagnetized rapidly. This permits a one second rise time for magnetizing the sample to 7 tesla, a two second flat top for heat exchange between the sample and working fluid, a one second discharge time and a two second hold time. For these requirements the limit on field is considered to be about 7 tesla for pulse superconducting magnets using a NbTi superconductor.
The arrangement of pulsed solenoid magnet and magnetocaloric sample described above has three disadvantages. First, the attainment of 8 tesla or higher field with pulsed superconducting magnets using a NbTi superconductor is not achievable in practice. Second, the magnetic energy on discharge (cycling) of the pulsed magnet is not recovered, thereby making the efficiency of the system very low. Lastly, the cycling rate from zero field to maximum field of 7 tesla is dependent on both the ability of the magnet to sustain large rates of change in a stable manner and on the rate at which the working fluid can be cooled down.